STudies are underway to identify neurotransmitters in photoreceptor cells and other retinal neurons. The objective of these studies is to understand the neurochemical circuitry responsible for processing visual information in retinas. Particular emphasis is given to the role of biogenic amines in visual systems. The visual system being studied currently is that of the American horseshoe crab Limulus polychemus. The anatomy and physiology of this visual system is well understood; virtually nothing is known of its neurochemistry. Studies conducted in this Laboratory have focused on the possible role of the putative neurotransmitters octopaine and serotonin. Biochemical techniques have been employed to study the synthesis, accumulation and metabolism of these amines. Electrophysiological techniques are being developed to study the role of these amines in visual function.